1. Field Of The Invention
The present invention relates to color image signal processing. More particularly, the invention relates to the construction of a look-up table for transforming imaging information between non-linearly related color spaces.
2. Description Of The Related Art
In the following description of relevant background art, reference is made to FIG. 1, of the accompanying drawings, which illustrates generally a problem sometimes associated with the constructing of a look-up table (LUT) for non-linearly related variables.
Color image reproduction systems known in the art permit an operator to edit the color and composition of a given image to form a reproduced image. For example, U.S. Pat. No. 4,500,919 discloses an image reproduction system of one type in which an electronic reader scans a color image, which may be in the form of a transparency or a print, and converts it to an electronic image. A computer workstation and an interactive operator interface, including a video display, permit an operator to edit the image displayed. When the operator has composed a desired image on the video display, the workstation causes an output writer device to make an inked output that is intended to match the reproduced image composed on the video display.
It is also known in the color image reproduction art to computer process color signals in digital form. Digital color correction can provide high accuracy, speed, and flexibility as well as relative immunity to noise. To those ends, U.S. Pat. No 4,346,402 teaches that color correction can be carried out by means of a digitally operated LUT for transforming color coordinate values from one color space to another color space.
A LUT in color image reproduction apparatus can serve to transform a set of color imaging data (multiple independent variables) in one color space to a corresponding set of color imaging data (one or more dependent variables) in a different color space. For example, by mixing three input sources (independent variables)--a red (R), a green (G), and a blue (B)--almost any spectrum output parameter (dependent variable) can be obtained, such as the Munsell value, hue and chroma; the 1976 CIE color spaces CIELAB (L*a*b*) and CIELUV (L*u*v*); etc.
The specifications for Patches of material in a given color space are commonly derived via measurement and calculation using color test strips. Having the specifications for an input color space, for example RGB, and corresponding specifications for an output color space, for example CIELUV (L*u*v*), the transform or functional relationship between the two color spaces can then be defined by means of storing corresponding independent/dependent data values in a LUT.
In the process of gathering test data to define a transformation, however, certain colors may not have been produced on the test strips. In some cases, this may simply be due to missing data; in other cases, however, the film used for the test strips is not capable of supporting the full range of colors to be defined by the LUT.
An input data set 10, illustrated diagrammatically in FIG. 1, used to generate a LUT, must be distributed in a way that the data supply information concerning all LUT nodal values, defined by the intersections of the horizontal lines 12 and the vertical lines 14. If there are "holes", denoted 16, in the data set 10, or if the data set does not "cover" the entire range of the LUT, as denoted by the peripheral region 18, it may not be Possible to construct a LUT for transforming information between non-linearly related variables. When, for example, a LUT is constructed by means of a least squares algorithm, as taught in the aforementioned U.S. patent application Ser. No. 259,548, the least squares technique may be unsolvable (singular). This can happen even if there are more data points in total than there are nodes defining the LUT.